This invention relates to techniques for providing uncorrupted communication between units of computer system that are interconnected by a cable.
Microcomputer systems, as it known to practically everyone, utilize a central computer or processing unit that is interconnected with a one or more other separate units by an appropriate electrical cable. Such other units can include various peripherals, such as printers, disk storage drives, and the like. It is, of course, important that various control, status, and data pulses are transmitted between the peripheral devices and the central computing unit, any corruption of the transmitted pulses being maintained at a level low enough so as not to affect operation of the computer system.
One such computer system is the Apple Macintosh brand. Its central computing unit includes a monitor, one or more disk drives, and several plugs (ports) for connection by cable with one or more separate peripheral units, such as a printer. One such port provided for this purpose follows an industry standard Small Computer System Interface, generally referred to as a "SCSI"standard. This standard designates the various conductors and plug pins of an 18-conductor cable for carrying various types of signals, the signal voltage levels, and similar parameters. The individual conductors are biased at a positive voltage, pulses being communicated along them by driving the voltage down. The central computing unit knows a pulse exists in a line by detecting its trailing, rising voltage edge. Eight conductors of the SCSI bus are designated as a data bus, another as a request line, yet another as an acknowledge line, with the remaining conductors providing various other control and status functions.
In operation of some Macintosh brand computer systems, it has been found that acknowledge signals are sent by the central computing unit over the acknowledge line of the interconnecting cable to a peripheral in response to something on the request line other than a proper request pulse generated by a peripheral. This can result in the central computing unit taking in false data from the data bus lines, or sending out such false data.
It is therefore a primary object of the present invention to provide a technique, and circuit implementation thereof, for minimizing or eliminating false requests from being acted upon by the central computing unit.